Car steering system

ABSTRACT

A steering system for a car having two front direction wheels and a number of electronic dynamic-performance control devices; the steering system has a steering wheel for controlling a turn angle of the front direction wheels, and a power-assist device which generates a power-assist torque which is added to the torque exerted on the steering wheel to vary the turn angle of the front direction wheels; the power-assist device has a control unit, which determines operation of the electronic dynamic-performance control devices, and modifies the power-assist torque as a function of operation of the electronic dynamic-performance control devices.

TECHNICAL FIELD

The present invention relates to a car steering system.

BACKGROUND ART

Most cars are now equipped with a power-assist or power steering devicefor reducing the torque exerted on the steering wheel to modify the turnangle of the front direction wheels.

The most widely used power-assist device is hydraulic, and comprises ahydraulic actuator, which generates a power-assist torque and iscontrolled by a number of valves activated by the angular position ofthe steering wheel.

The major drawback of power-assist devices of the above type lies in thepower-assist torque being constant, and so tending to “overslacken” thesteering wheel at high speed, so that the driver is unable to actuallyfeel the dynamic performance of the car. To eliminate this drawback, ithas been proposed to equip the power-assist device with aspeed-sensitive valve, which gradually reduces the power-assist torqueas speed increases.

Even a power-assist device with a speed-sensitive valve, however, doesnot allow the driver to accurately feel the dynamic performance of thecar, especially when the car is equipped with electronicdynamic-performance control devices which greatly reduce the reactionsof the car. For example, an ASR device prevents spinning of the drivewheels; an ABS (Anti Block System) and MSR device prevent the wheelslocking when braking; an E-diff (electronic differential lock percentagecontrol) device and an ESP (Electronic Stability Program) device limitswerving and excessive load transfer.

DE10245975A1discloses a controlled planetary steering transmission withhydraulic rack assistance. The steering system comprises a hydraulicassistance computing unit controlling a valve 2 in response to signalsincluding sensed input torque, and a steering wheel delivering the inputtorque to a portion of a steering column, from which rotation istransmitted via epicyclic gearing and a pinion to the rack.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a car steeringsystem that is cheap and easy to produce, eliminates the aforementioneddrawbacks, and, in particular, allows the driver to actually feel thedynamic performance of the car.

According to the present invention, there is provided a car steeringsystem as claimed in the accompanying Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A non-limiting embodiment of the present invention will be described byway of example with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic of a car equipped with a steering system inaccordance with the present invention;

FIG. 2 shows a schematic of a hydraulic power-assist device of the FIG.1 steering system.

PREFERRED EMBODIMENTS OF THE INVENTION

Number 1 in FIG. 1 indicates a rear-drive car comprising two frontdirection wheels 2 and two rear drive wheels 3. The turn angle of frontdirection wheels 2 is controlled by a steering system 4 comprising asteering arm 5, which extends crosswise to car 1, is hinged at oppositeends to both front direction wheels 2, and is movable axially by asteering wheel 6. More specifically, steering wheel 6 is fitted to arotary steering shaft 7 connected mechanically to steering arm 5 by arack-and-pinion coupling device 8.

Steering system 4 also comprises a hydraulic power-assist (so-called“power steering”) device 9, which generates a variable force on steeringarm 5 to reduce (or, more generally speaking, modify) the torque exertedon steering wheel 6 to modify the turn angle of front direction wheels2.

As shown in FIG. 2, hydraulic power-assist device 9 comprises ahydraulic actuator 10 located at an intermediate portion of steering arm5, and which comprises two chambers 11 filled alternately with apressurized fluid to move steering arm 5 axially in both directions.More specifically, the two chambers 11 are fitted through with steeringarm 5, are located in series along steering arm 5, and are separated bya flange 12, which is integral with steering arm 5, defines a piston ofhydraulic actuator 10, and comprises a central annular cavity 13 housingan annular seal 14.

Hydraulic power-assist device 9 also comprises a storage tank 15containing the control fluid (typically oil) of hydraulic actuator 10 atambient pressure; a hydraulic accumulator 16 containing pressurizedcontrol fluid; a motor-driven pump 17, which draws control fluid fromstorage tank 15 and pumps pressurized control fluid to hydraulicaccumulator 16; and two proportional solenoid valves 18 for selectivelyconnecting chambers 11 of hydraulic actuator 10 to storage tank 15 andhydraulic accumulator 16. More specifically, each chamber 11 ofhydraulic actuator 10 is associated with a respective three-way solenoidvalve 18, which isolates chamber 11 to maintain a constant amount ofcontrol fluid inside chamber 11, connects chamber 11 to storage tank 15to drain the control fluid from chamber 11, or connects chamber 11 tohydraulic accumulator 16 to feed control fluid into chamber 11.

Finally, hydraulic power-assist device 9 comprises a damp valve 20located between the two chambers 11 to prevent any fluctuations inpressure (hammering); and a safety valve 21, which, in the event of abreakdown, connects both chambers 11 permanently to storage tank 15 bymeans of respective bypass conduits 22.

Hydraulic accumulator 16 is fitted with a pressure sensor 23 fordetermining the pressure of the fluid inside hydraulic accumulator 16;each chamber 11 is fitted with a pressure sensor 24 for determining thepressure of the fluid inside chamber 11; and pressure sensors 23 and 24are connected to a control unit 25 for controlling power-assist device9. More specifically, hydraulic actuator 10 is feedback-controlled bycontrol unit 25 using the fluid pressures detected in chambers 11 bysensors 24 as feedback variables.

Car 1 is equipped with a number of electronic dynamic-performancecontrol devices 26, which act on various active components (e.g. theengine and brakes) of car 1 to alter the dynamic performance of car 1.More specifically, electronic dynamic-performance control devices 26comprise an ASR device for preventing spinning of rear drive wheels 3;an ABS (Anti Block System), which acts on the brake system to preventlocking of wheels 2 and 3 when braking; an MSR device, which acts on thedrive torque to prevent rear drive wheels 3 from locking when braking;an E-diff (electronic differential lock percentage control) device, andan ESP (Electronic Stability Program) device, which limit swerving andexcessive load transfer.

Control unit 25 is connected to electronic dynamic-performance controldevices 26 (e.g. over a BUS of car 1) to real-time determine the statusof electronic dynamic-performance control devices 26. In other words,control unit 25 is able to determine whether, and to what extent, anelectronic dynamic-performance control device 26 is operating.

Control unit 25 is also connected to sensors 27 for real-timedetermining longitudinal speed Vx, longitudinal acceleration Ax,transverse (or lateral) acceleration Ay, and the swerve angle of car 1.

In a preferred embodiment, control unit 25 is also connected to a torquesensor 28 fitted to steering shaft 7 to determine the torque exerted bythe user on steering wheel 6.

In actual use, control unit 25 determines operation of electronicdynamic-performance control devices 26, and modifies the power-assisttorque accordingly.

When the ESP device indicates car 1 is close to its road-holding limit,control unit 25 may increase the power-assist torque to alert the driveraccordingly in advance by increasing natural “slackening” of steeringwheel 6 in advance. This control mode is better suited tonon-professional drivers, whose main concern is safety, as opposed topushing the car to its extreme limit. Conversely, when the ESP deviceindicates car 1 is close to its road-holding limit, control unit 25 mayreduce the power-assist torque to counteract natural “slackening” ofsteering wheel 6. This control mode is better suited to professionaldrivers, whose main concern is pushing the car to its extreme limit, asopposed to safety.

On determining operation of the E-diff (electronic differential lockpercentage control) device, control unit 25 may introduce into thepower-assist torque a pulsating (i.e. time-variable) component of afrequency discernible by the driver (e.g. 1-5 Hz) and of low enoughintensity (5-10% of the total) not to affect driving. The purpose of thepulsating component is to alert the driver to operation of the E-diffdevice, and to the fact that car 1 is nearing a limit, both of which,without the pulsating component, could go unnoticed by the driver(particularly a non-professional driver).

By means of sensors 27, control unit 25 also determines speed Vx,accelerations Ax and Ay, and the swerve angle of car 1, and modifies thepower-assist torque accordingly.

Finally, by means of sensor 28, control unit 25 determines the torqueexerted by the driver on steering wheel 6, and modifies the power-assisttorque accordingly.

In other words, the final power-assist torque exerted on steering arm 5depends on operation (if any) of electronic dynamic-performance controldevices 26, on speed Vx, accelerations Ax and Ay, and the swerve angleof car 1, and on the torque exerted by the driver on steering wheel 6.In a preferred embodiment, the final power-assist torque exerted onsteering arm 5 is largely determined (80-90%) on the basis of speed Vxof car 1, and is corrected slightly (10-20%) as a function ofaccelerations Ax and Ay of car 1, the swerve angle of car 1, operation(if any) of electronic dynamic-performance control devices 26, and thetorque exerted by the driver on steering wheel 6.

As will be clear from the above description, hydraulic power-assistdevice 9 provides for “variable torque feedback” on steering wheel 6 ofcar 1. The variation in feedback is regulated by control unit 25 bymeans of hydraulic actuator 10, and is determined on the basis of thecauses (e.g. ABS, MSR, E-diff, ESP intervention) and effects (speed,turn angle, accelerations, swerve angle) affecting dynamic performanceof car 1.

1. A steering system for a car having two front direction wheels and a plurality of electronic dynamic-performance control devices including an ESP device for controlling stability of the car; the steering system comprising: a steering wheel for controlling a turn angle of the front direction wheels; and a power-assist device, which generates a power-assist torque which is added to the torque exerted on the steering wheel to vary the turn angle of the front direction wheels; wherein the power-assist device comprises a control unit, which determines operation of the electronic dynamic-performance control devices, and modifies the power-assist torque as a function of operation of the electronic dynamic-performance control devices; wherein the control unit comprises a first control mode for non-professional drivers whose main concern is safety as opposed to pushing the car to its extreme limit, in which, when the ESP device indicates the car is close to its road-holding limit, the control unit increases the power-assist torque to alert the driver accordingly in advance by increasing natural “slackening” of the steering wheel in advance; and wherein the control unit comprises a second a control mode for professional drivers whose main concern is pushing the car to its extreme limit, as opposed to safety, in which, when the ESP device indicates the car is close to its road-holding limit, the control unit reduces the power-assist torque to counteract natural “slackening” of the steering wheel.
 2. The steering system as claimed in claim 1, wherein the electronic dynamic-performance control devices comprise an ASR device for preventing spinning of the drive wheels; an ABS device, which acts on the brake system to prevent locking of the wheels when braking; an MSR device, which acts on the drive torque to prevent the drive wheels from locking when braking; and an E-diff device for electronically controlling the lock percentage of a differential.
 3. The steering system as claimed in claim 1, wherein the control unit is connected to the electronic dynamic-performance control devices over a BUS of the car to real-time determine the status of the electronic dynamic-performance control devices.
 4. The steering system as claimed in claim 1, wherein, when the control unit determines operation of an E-diff device for electronically controlling the lock percentage of a differential, the control unit introduces into the power-assist torque a pulsating component at a frequency discernable by the driver and of low intensity.
 5. The steering system as claimed in claim 1, wherein the control unit is connected to sensors for real-time determining the longitudinal speed Vx, longitudinal acceleration Ax, transverse acceleration Ay, and swerve angle of the car; the control unit determines the speed Vx, accelerations Ax and Ay, and swerve angle of the car by means of the sensors, and modifies the power-assist torque as a function of the speed Vx, accelerations Ax and Ay, and swerve angle of the car.
 6. The steering system as claimed in claim 1, wherein the control unit is connected to a torque sensor connected to a steering shaft to determine the torque exerted by the driver on the steering wheel; the control unit determines the torque exerted by the driver on the steering wheel by means of the torque sensor, and modifies the power-assist torque as a function of the torque applied by the driver on the steering wheel.
 7. The steering system as claimed in claim 1, wherein the power-assist device comprises a hydraulic actuator located at an intermediate portion of the steering arm, and which comprises two chambers filled alternately with a pressurized fluid to move the steering arm axially in different directions.
 8. The steering system as claimed in claim 7, wherein the two chambers are operably engaged with the steering arm, are arranged in series along the steering arm, and are separated by a flange, which is integral with the steering arm and defines a piston of the hydraulic actuator.
 9. The steering system as claimed in claim 8, wherein the power-assist device comprises a storage tank containing the control fluid of the hydraulic actuator at ambient pressure; a hydraulic accumulator containing pressurized control fluid; a motor-driven pump which draws control fluid from the storage tank and pumps pressurized control fluid to the hydraulic accumulator; and two proportional solenoid valves for selectively connecting the chambers of the hydraulic actuator to the storage tank and the hydraulic accumulator.
 10. The steering system as claimed in claim 9, wherein the power-assist device comprises a damp valve located between the two chambers to prevent fluctuations in pressure.
 11. The steering system as claimed in claim 9, wherein the power-assist device comprises a safety valve, which, in the event of a breakdown, connects the two chambers permanently to the storage tank by means of respective bypass conduits.
 12. The steering system as claimed in claim 9, wherein the hydraulic accumulator is fitted with a first pressure sensor for determining the pressure of the fluid in the hydraulic accumulator; and each chamber is fitted with a second pressure sensor for determining the pressure of the fluid in the chamber.
 13. The steering system as claimed in claim 12, wherein the control unit controls the power-assist device, and is connected to the second pressure sensors; and the hydraulic actuator is feedback-controlled by the control unit using the fluid pressures detected inside the chambers by the second pressure sensors as feedback variables. 